This application is a continuation-in-part of Ser. No. 813,244 filed Dec. 24, 1985 which is a continuation-in-part of Ser. No. 810,060, filed Dec. 17, 1985, U.S. Pat. No. 4,725,958.
The invention disclosed herein is an improvement to the inventor's prior automatic pellet mill controllers as disclosed and claimed in U.S. Pat. Nos. 3,932,736, 4,340,937 and 4,463,430, all of which were invented by the inventor herein and which are commonly owned by the assignee of the present invention. These prior patents are incorporated herein by reference.
The automatic pellet mill controllers disclosed in these prior patents essentially sense the temperature of the material as it is processed by the pelleting apparatus and at various points therealong, measures the difference between two of those sensed temperatures, and controls the operation of the pelleting apparatus by controlling the rate of addition of ingredients thereto. Of course, each of these systems has its ow unique features and reference should be made directly to those prior patents for complete details of each system.
The temperature of the material is sensed at various points in the pelleting apparatus through the use of direct contact temperature sensors, such as 106 or 109 as shown in the '937 patent which requires direct physical contact between the material and the temperature sensor to achieve an accurate measurement thereof. In the prior art, temperature sensors are available with stainless steel dual walled shielding to withstand the abrasive effect of the granular material as it rubbed against the temperature sensor. Although these temperature sensors do perform adequately, they represent an on-going maintenance requirement necessitating periodic checking and replacement thereof to ensure continuous, satisfactory pelleting. Furthermore, at lower levels of production and for runs producing large diameter pellets, there is an opportunity for air to partially surround the probe instead of material because there is an insufficient mass of material flowing through the system. In addition, due to physical difficulties in locating the probe, the pellets lose temperature and the indicated temperatures are sometimes not representative of the actual temperatures of the pellets as they exit the die. This results in some inaccuracies in temperature measurement which, if not accounted for, can detract from the ability of the control to maximize production throughput and pellet quality.
To solve these and other difficulties encountered with using direct contact temperature probes in the prior art, the inventor herein has succeeded in adapting a non-contacting, IR sensing temperature sensor to the pellet mill controllers which senses the temperature of the material and uses that parameter in controlling the pellet mill. The inventor has selected a Raytec.TM. Thermalert II.TM. Model No. T2L2 which senses the infrared frequencies emanating from the material to obtain a very accurate temperature reading of the material. Replacement of the direct contact temperature sensors of the inventor's prior systems, or for that matter other systems, eliminates the problems experienced therein. Additionally, the inventor has also discovered that there are advantages to sensing the temperature of the pellets as they are formed by the rotating die within the mill and using that temperature in combination with the temperature of the material as it enters the mill, much as in the manner of the .DELTA.T mill control disclosed in the '937 patent mentioned above. When the mill is first started up, the die is cold, and its temperature is rapidly changing until pellets are formed by the die and their temperature measured, Therefore, directly sensing the temperature of the pellets at the die gives a direct indication of the temperature at which the first pellets will be formed, and will also directly follow the heating up of the die as the pelleting run continues. This provides for a smoother start-up of each pelleting run, and more accurate control of the pelleting process to produce good quality pellets even at the beginning of a run.
In addition to its use in combination with a second temperature sensor to generate a T measurement for use in the control algorithms as disclosed in the parent applications and prior patents mentioned above, the inventor has determined that the single point temperature measurement of the pellets at the die can be used to control the moisture input as a new control algorithm. In most pelleting apparatuses, the pellets are formed by a mixture of the material being forced through holes in the die face and then cut off by one or more blades such that the temperature of the die closely approaches that of the pellets as they are formed. In fact, when the process is at a steady state temperature, the temperature of the die is almost exactly the same as the temperature of the pellets as they emerge from the face of the die after being squeezed through the holes therein. However, when the process is undergoing a change in temperature due to an adjustment made by the controller or due to a change in the materials being pelleted, the die temperature may increase over that of the pellet temperature as the mixture being pelleted contains moisture which absorbs the heat by converting more moisture into steam. Therefore, at some time during the pelleting process, the die temperature may be somewhat elevated over that of the pellet temperature. The non-contacting temperature probe of the present invention can compensate for this somewhat by taking an average reading through its bore sight which can cover not only pellets as they emerge, but also portions of the die face. Nonetheless, the die temperature, or the temperature of pellets immediately upon their emergence from the die face, is a measurement taken at exactly the point of pellet formation, a temperature which has been found to be useful and sufficient of itself to control the pelleting process.
The inventor has found that controlling this sensed die temperature, or pellet temperature immediately upon pellet emergence from the die face, or some average therebetween, by controlling the input of moisture into the pelleting apparatus results in finished pellets which have a moisture content within a prescribed range. If pellets are being produced which have a moisture content which is too high, the desired die temperature can be elevated. The control achieves the higher die temperature by adding less moisture to the process to increase friction in the die, or by increasing the average temperature of the moisture being added. This can be achieved by adding more steam at approximately 225.degree. F. and less hot water at approximately 160.degree. F. With less moisture, pellets having a reduced moisture content are produced and the finished pellets' moisture content is brought down. With higher moisture temperature, more moisture is boiled off during pelleting resulting in finished pellets with a lower moisture content. If pellets are produced having too low a moisture content, then the desired die temperature can be reduced. The control achieves the lower die temperature by adding in increased moisture to decrease the frictional forces in the die thus producing finished pellets with a somewhat greater moisture content. Alternately, moisture with a lower average temperature can be added. The inventor has found that for some materials, this control algorithm provides optimum results and hence represents another mode of control in addition to .DELTA.T and .DELTA.T mill, as disclosed in the prior patents mentioned above.
As a result of finally being able to accurately sense the temperature of pellets at their point of formation, the inventor has further discovered that pelleting at a nominal 212.degree. F. seems to result in optimal performance for the pellet mill. More specifically, the desired operating temperature appears to be that temperature at which water changes physical state into steam, a temperature which is dependent upon the local atmospheric pressure. It is believed that at this nominal boiling point, optimal pellets are formed in that there is a more complete cooking of the food materials, and the presence of surplus water in the mixture provides a cushioning effect to help compensate for variations of the material being pelleted and the instantaneously changing frictional forces encountered in the mill. It has been found that pelleting at temperatures above a nominal 227.degree. F. results in a very high probability of choking the mill. This is thought to be caused by the fact that each added BTU of energy is absorbed directly by the material itself, there being virtually no water left to absorb the energy as it is transformed into steam. This makes the pelleting process at this temperature quite unforgiving for variations in the material being pelleted and the frictional forces generated in the mill.
By utilizing the non-contacting temperature probe to sense the temperature of the pellets as they emerge from the die, a method of pelleting can be utilized which holds that pellet temperature to the temperature at which water will boil, or a nominal 212.degree. F. This control mode was heretofore unavailable because of the inability of any of the prior art devices to accurately sense the instantaneous pellet formation temperature. By utilizing this method, significantly improved operating results can be achieved and the mill operation can also be made less sensitive to minor variations in temperature, material, and the frictional forces encountered in the mill as the pelleting occurs. This furthermore has the advantage of ensuring a complete cooking of the material to achieve a satisfactory pellet.
It should be noted that although the inventor has used the control algorithm of die temperature to control the addition of moisture, it is within the range of equivalents to implement the die temperature control algorithm to control the addition of any ingredient with a significant moisture content as would be appropriate and as would have a sufficient effect upon the moisture content of the finished pellets to produce the desired results. For example, instead of controlling water input, this control algorithm could be used to control or vary the input of a wet grain at the same time that a dry grain is input to the apparatus. Thus, controlling the relative mixture between the two would also serve to control the moisture content of the mixture to be pelleted and achieve the desired results. Still other processes would achieve lubrication with materials other than moisture such as fats, oils, or the like. It is believed that this same control algorithm will serve to control those processes by controlling the amount of relative lubrication present in the mixture to be pelleted, whether that lubrication is provided by moisture or one of these other ingredients. It is also contemplated that the pelleting of products that have materials that boil or change state at other temperatures like alcohol, gasoline, salt water, or plastic could be controlled in a similar manner, at or near the temperature at which there is a change of phase. Therefore, it is intended that these systems be included and covered as well.
To enhance the die temperature control algorithm, the inventor has added a moisture meter to measure the moisture of finished pellets after they have exited the cooler, and used the moisture meter output to automatically adjust the desired die temperature. In most processes, it has been found that the moisture content of the pellets changes as they are processed through the cooler. In most instances, the moisture content of the pellet after it has cooled is the parameter most desired to be controlled. This is because tests have been conducted on finished cooled pellets to determine a prescribed moisture content to optimize pellet durability and decrease fines produced in the process. As cooling is one step in the process, its effect on the pellet should not be ignored in controlling the process to produce finished pellets having a desired or prescribed moisture content. By adding the moisture meter at the cooler to sense finished pellet moisture content, and automatically adjusting the desired die temperature, the effect of cooling on finished pellets can be taken into account. This represents a significant advantage over those systems known in the prior art which do not adjust the pelleting process for the effects of cooling on the moisture content of finished pellets.
The foregoing has been a brief summary description of some of the principal advantages and features of the present invention. A greater understanding and appreciation for further details of the invention may be obtained by referring to the drawings and description of the preferred embodiment which follow.